Agriculture Reference
In-Depth Information
against observed data from multiple ield sites yields conidence in recommended
values for parameterization and predictive capability. A preference is given
towards continuous simulation models, which also means further reinement of
living roof ET models is required. The elements of a future ideal design process
include:
1 Use a veriied hydrologic model(s) to predict stormwater retention and deten-
tion. The model determines the moisture holding characteristics needed to
satisfy (at a minimum) objectives for stormwater control, according to project-
speciic climate considerations.
2 Test the candidate growing media to ensure that the moisture holding charac-
teristics of the candidate materials satisfy the assumptions of the model.
3 Review outcomes with the design consultant team to determine if the result-
ant assembly design for stormwater control is feasible according to allowable
structural loading, and also supports plant health and biodiversity objectives
while considering the reliability of long-term maintenance.
4.1.2.2 Saturated hydraulic conductivity or permeability
In common language, the terms iniltration , permeability and hydraulic conduc-
tivity tend to be used synonymously. Permeability is a non-speciic term. Techni-
cally, iniltration refers to the rate at which water enters through the surface of a
media (Fredlund and Rahardjo 1993), whereas hydraulic conductivity is the ease
with which water moves through soil pores (measured as the rate at which water
percolates through the media itself) (NRCS 2004). Saturated hydraulic conductiv-
ity is a standard engineering or soil science assessment that quantiies this rate
when all pore space is illed with water. In most storms, engineered media is typi-
cally unsaturated. Unsaturated hydraulic conductivity varies with moisture
content, but is nearly always less than saturated hydraulic conductivity (the latter
includes large pores typically illed with air under unsaturated conditions). Satu-
rated hydraulic conductivity tends to be the preferred metric for predicting the
potential for surface ponding in living roof systems. In FLL (2008) or ASTM
E2399-11 terms, the term “water permeability” is used, but the test itself relects
a falling head methodology for saturated hydraulic conductivity more widely
known in civil engineering.
Ensuring adequate low capacity through the growing media is designed to
prevent water low across the surface of the growing media and plants, bypass
or surface ponding. Surface ponding adds excess weight, and may loat and/or
scour, or otherwise erode the growing medium if moving laterally across the
surface. For stormwater living roofs, a well-designed growing medium should
not pond water on the surface, and is unlikely to ever reach saturation (where
all pore space is occupied by water, Figure 2.2 ), unless vertical drainage points
are blocked or outlets are otherwise obstructed. In cold climates, plants, the
growing media and the roof structure are protected from potentially damaging
physical movement induced by freeze-thaw cycles by freely draining media (as
 
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